[itex]I = I_{center} + md^{2}[/itex]
[itex]I = mr^{2}/2 + mr^{2}[/itex] [Since the disk rotates about an axis through the edge, we must add the inertia by mr². r is the distance between the center and the edge of the disk.]
[itex]I = 3mr^{2}/2[/itex]

Is that how I approach this? Let me know where I go wrong. Otherwise, I can just plug and chug this expression: